12 April, 2014

Making sense of shark senses

If you have
ever been asked, which sense is the most important sense to sharks? Keep
reading…

The good people at Mote Marine Lab have done it again. They seem to always crank out the good stuff, but this study is simply exemplary. Here’s an
overly simplified version of the bits I liked the most – for all the bits, go
read it yourself!

They collected several individuals of three
species of shark - blacktips C.limbatus,
bonnetheads S.tiburo, and nurses G.Cirratum. They also collected live prey items from the
diets of each (blacktip+nurse = pinfish L.rhomboides
and bonnethead = pink shrimp F.duorarum). Each species’ general 'norm' of how they go
about detecting, tracking, orientating, striking and capturing their prey was
established. Researchers then blocked some of
four senses (smell, vision, electroreception, lateral line) one at a time or in
combination and observed how the individuals adapted to these changes and
shifted their behaviours from the established ‘norm’ of all senses firing.

When it
comes to the initial detection of prey, the
sense of smell reined king for all three species. While blacktips and bonnetheads would still capture
the prey when they eventually came into visual contact with it, they did not
detect and track the prey from a distance as they would normally. For nurse sharks, sense of smell was an
absolute necessity. With their nares
blocked, nurse sharks sat at the bottom of the tank and failed to feed
entirely. When smell and vision were blocked, blacktips and bonnethead also failed to
feed. This, of course, makes sense. Nurse sharks primarily hunt at night or hunt
prey animals hidden from view in reefs, whereas blacktips and bonnetheads have
light available to them. So guess who
performed the best when vision and lateral line were blocked? Yep, the nurse shark. Who did the worst? Bonnetheads.

The sense of
electroreception was linked with successfully capturing prey. When blocked, the sharks could still detect,
track, orientate and strike at the prey, but would simply forget to open their
jaws in time to capture it, even if the prey actually touched the shark!

Source. A. A bonnethead,Sphyrna tiburo, with all senses intact opens the mouth to capture shrimp using ram-biting. B. The same bonnethead fails to open the mouth when electroreception is blocked and misses the shrimp, despite making tactile contact with the prey. doi:10.1371/journal.pone.0093036.g006

Vision and
lateral line seemed to go hand in hand: “…animals
with simultaneous vision and lateral line blocks did not orient or strike, even
when they were within electrosensory prey detection range.” Thus, “these results suggest that sharks do
not recognize electrical cues alone and prey, but require an additional visual
or olfactory cue.”

In conclusion:

Our results demonstrate that sharks are capable of attending to multiple sensory cues simultaneously, switching sensory modalities in a hierarchical fashion as they approach their prey, and substituting alternate sensory cues, when necessary, to accomplish behavioural tasks. This flexibility in behaviour suggests that sharks are well adapted to success, even in the face of a changing environment and evolutionary advancements in prey defenses including chemical, visual, and mechanical camouflage. Gardiner et al. 2014

The bottom
line? Sharks are not the super simple
one-sensory fish that some may want you to believe that they are. Elegant behavioural research, bravo!

It would be
very interesting, but impossible, to do a similar study on large migratory sharks to see
what senses they may use to navigate over long (say, Dyer Island to Mozambique) distances, but alas...

They blocked the electrosense by painting a particular substance over the ampullae of Lorenzini and blocked the lateral line sense by soaking sharks in another particular chemical substance. Both of these would eventually wear off the shark and they would regain their sense (check the actual paper and under "Methods" it says the exact chemicals used). So, not a practical shark deterring method, and as the paper says, even with both of these blocked, sharks could still sniff out and search out potential prey.